Federal law states 40 hours per week, no limit on the number of hours per day.
In Hawaii, when I worked for an inventory service, sometimes we would work for 12-16 hours straight with outside breaks because we were literally locked in the store. The busy season was January and some people would for days without sleep, going from job to job.
When I worked at the print shop at OfficeMax, I’d sometimes work for over 24 hours straight when we had big print jobs, only getting a lunch and dinner break. I was paid OT only when I went over 40 hours for the week.
For contracted jobs, the contract may specify number that OT be paid for work over 8 hours as well as double time for holiday pay. IIRC, this was usually for State projects. I also know for certain that on Federal projects, no one was allowed to work on Labor Day.
The federal overtime provisions are contained in the Fair Labor Standards Act (FLSA). Unless exempt, employees covered by the Act must receive overtime pay for hours worked over 40 in a workweek at a rate not less than time and one-half their regular rates of pay. There is no limit in the Act on the number of hours employees aged 16 and older may work in any workweek. The Act does not require overtime pay for work on Saturdays, Sundays, holidays, or regular days of rest, unless overtime is worked on such days.
The Act applies on a workweek basis. An employee’s workweek is a fixed and regularly recurring period of 168 hours — seven consecutive 24-hour periods. It need not coincide with the calendar week, but may begin on any day and at any hour of the day. Different workweeks may be established for different employees or groups of employees. Averaging of hours over two or more weeks is not permitted. Normally, overtime pay earned in a particular workweek must be paid on the regular pay day for the pay period in which the wages were earned.
https://www.dol.gov/agencies/whd/overtime
However, State law may specify an 8 hour day. For example in California, with exemptions:
[edit: ninjaed by Snowboard_Bo]
In California, the general overtime provisions are that a nonexempt employee 18 years of age or older, or any minor employee 16 or 17 years of age who is not required by law to attend school and is not otherwise prohibited by law from engaging in the subject work, shall not be employed more than eight hours in any workday or more than 40 hours in any workweek unless he or she receives one and one-half times his or her regular rate of pay for all hours worked over eight hours in any workday and over 40 hours in the workweek (or double time as specified below). Eight hours of labor constitutes a day’s work, and employment beyond eight hours in any workday or more than six days in any workweek requires the employee to be compensated for the overtime at not less than:
1. One and one-half times the employee’s regular rate of pay for all hours worked in excess of eight hours up to and including 12 hours in any workday, and for the first eight hours worked on the seventh consecutive day of work in a workweek; and
2. Double the employee’s regular rate of pay for all hours worked in excess of 12 hours in any workday and for all hours worked in excess of eight on the seventh consecutive day of work in a workweek.
There are, however, a number of exemptions from the overtime law. An “exemption” means that the overtime law does not apply to a particular classification of employees. There are also a number of exceptions to the general overtime law stated above. An “exception” means that overtime is paid to a certain classification of employees on a basis that differs from that stated above. In other words, an exception is a special rule. (For special rules regarding overtime for agricultural workers, please see Overtime for Agricultural Workers.)
https://www.dir.ca.gov/dlse/faq_overtime.htm